1. Field of the Invention
This invention relates to a voltage controlled oscillator (VCO), and more particularly to a VCO used as a local oscillator of a PLL (Phase Locked Loop) circuit.
2. Description of the Related Art
FIG. 1 shows an example of the construction of the conventional VCO. The oscillator is built in "YM3623" (Digital Audio Interface Receiver) made by NIHON GAKKI SEIZOU KABUSHIKI KAISHA and is disclosed in the catalog No. LSI-1136230, p. 5.
The VCO shown in FIG. 1 has n-stage (n is an odd number) delay circuits 11-1 to 11-n connected in a ring form. As is represented by the delay circuit 11-1, each of the delay circuits 11-1 to 11-n includes P-channel MOS transistors P1, P2 and N-channel MOS transistors N1, N2. The MOS transistors P2 and N2 constitute a complementary inverter, the current path of the MOS transistor P1 is connected between the source of the MOS transistors P2 and a power supply V.sub.DD, and the current path of the MOS transistor N1 is connected between the source of the MOS transistors N2 and a ground terminal GND. The gate of the MOS transistor P1 is connected to the ground terminal GND and the gate of the MOS transistor N1 is applied with a control voltage Vc. The output node of the first stage delay circuit 11-1 is connected to the input node of the second stage delay circuit 11-2, the output node of the second stage delay circuit 11-2 is connected to the input node of the third stage delay circuit 11-3, and in the same manner, the outputs of the delay circuits 11-3 to 11-(n-1) are respectively connected to the input nodes of the next stage delay circuits 11-4 to 11-n. The output node of the n-th stage delay circuit 11-n is connected to the input node of the first stage delay circuit 11-1. An oscillation output OSC is generated from the n-th stage delay circuit 11-n.
If the delay time of each of the delay circuits 11-1 to 11-n shown in FIG. 1 is Td, the oscillation frequency f.sub.osc. of the oscillator is expressed by the following equation. EQU f.sub.osc. =1/(Td.times.n)
where n indicates the number of stages of the delay circuits.
The delay time Td depends on the time constant .tau..sub.up (=C.times.(rP1+rP2)) determined by the ON resistances rP1, rP2 of the MOS transistors P1, P2 and the parasitic capacitance C of the next stage gate capacitor or the like at the time of rise and depends on the time constant .tau..sub.down (=C.times.(rN1+rN2)) determined by the ON resistances rN1, rN2 of the MOS transistors N1, N2 and the parasitic capacitance C at the time of fall. Therefore, the following relation can be obtained. EQU Td.varies.(.tau..sub.up +.tau..sub.down)/2
If the control voltage Vc is changed, the ON resistance rN1 of the MOS transistor N1 is changed and the delay time Td is changed, thereby making it possible to change the oscillation frequency f.sub.osc..
FIG. 2 shows the oscillation characteristic of the oscillator shown in FIG. 1. As shown in FIG. 2, the oscillation frequency f.sub.osc. can be changed by the width of .DELTA.W by changing the control voltage Vc.
However, in the conventional oscillator as shown in FIG. 1, since the oscillation frequency f.sub.osc. is controlled only by use of the MOS transistor N1, the oscillation is made unstable by the unbalance between the time constants .tau..sub.up and .tau..sub.down particularly when the control voltage Vc becomes low and the MOS transistor N1 is turned OFF and the oscillation is interrupted when the control voltage is further lowered. Further, the stability of the oscillation frequency f.sub.osc. depends on the inclination of the control voltage Vc-oscillation frequency f.sub.osc. characteristic curve shown in FIG. 2 and the stable oscillation can be attained when the inclination is small. Therefore, in order to attain the stable oscillation in the conventional VCO, the variable range of the oscillation frequency cannot be increased to a large extend. Further, in a case where the VCO is used in an application system which is made unstable or erroneously operated when the oscillation is interrupted, it is necessary to make such a circuit design that the control voltage Vc will not become lower than a preset voltage.
For this reason, when the conventional VCO is used as a core cell for automatic design such as a semi-custom LSI, for example, the range of the application system may be limited by the limitation of the variable frequency and, thus, a limitation on the application thereof, caused by the fact that the oscillation is interrupted when the control voltage becomes excessively low, may be imposed. Therefore, it cannot be used as a core cell when high flexibility is required.
As described above, the conventional VCO cannot be applied to various types of application systems since the variable range of the oscillation frequency is narrow and the oscillation frequency which is stable in a range from the low frequency to the high frequency cannot be obtained. Further, the oscillation is interrupted when the control voltage is set to the ground potential and it is difficult to easily apply the VCO to the application system.